Tuning supramolecular structuring at the nanoscale level : Nonstoichiometric soluble complexes in dilute mixed solutions of alginate and lactose-modified chitosan (chitlac)

Two oppositely charged polysaccharides, alginate and a lactose-modified chitosan (chitlac), have been used to prepare dilute binary polymer mixtures at physiological pH (7.4). Because of the negative charge on the former polysaccharide and the positive charge on the latter, polyanion-polycation complex formation occurred. A complete miscibility between the two polysaccharides was attained in the presence of both high (0.15 M) and low (0.015 M) concentrations of simple 1:1 supporting salt (NaCl), as confirmed by turbidity measurements; phase separation occurred for intermediate values of the ionic strength (I). The binary solutions were further characterized by means of light scattering, specific viscosity, and fluorescence quenching measurements. All of these techniques pointed out the fundamental role of the electrostatic interactions between the two oppositely charged polysaccharides in the formation of nonstoichiometric polyelectrolyte soluble complexes in dilute solution. Fluorescence depolarization (P) experiments showed that the alginate chain rotational mobility was impaired by the presence of the cationic polysaccharide when 0.015 M NaCl was used. Moreover, upon addition of calcium, the P values of the binary polymer mixture in 0.015 M NaCl increased more rapidly than that of an alginate solution without chitlac, suggesting an efficient crowding of the negatively charged alginate chains caused by the polycation.